Patent GB-421691-A (GB)
Title: Improvements in and relating to the production of solid formaldehyde
IPC: C08G2/08
Filed: 1933-11-23
Abstract: Solid soluble modifications of formaldehyde are manufactured by subjecting a solution of formaldehyde to slow cooling of the order of about 1--3 DEG  C. per hour, the solid product formed being separated from the mother liquor and dried.  The formaldehyde solutions should advantageously be as concentrated as possible, i.e. 60--70 per cent, and should contain water at least in small quantities.  Suitable solutions may be obtained by the azeotropic removal of water from aqueous solutions of formaldehyde with a sufficient excess of auxiliary substance according to Specification 375,744; or by concentrating at fairly high temperatures ordinary commercial formaldehyde solutions by passing formaldehyde gas into them; or by extracting ordinary aqueous formaldehyde with extracting agents, e.g. ethyl acetate, or by condensing vapours containing formaldehyde formed by the oxidation of methyl alcohol, advantageously with a conversion of at least 70 per cent, and increasing the concentration of the condensate by withdrawal of water or addition of concentrated formaldehyde. The cooling, which may be over the range of about 65 DEG  C. to about 15 DEG  C. in 16--24 hours, may be regulated by transferring the solution into a suitably insulated vessel, or by cooling the crystallizing vessel by water whose temperature decreases by about 1--3 DEG  C. per hour, and is interrupted as soon as the rate falls below 1 DEG  C. per hour.  It is advantageous to add to the hot solution small quantities of electrolytes, particularly compounds of basic action, e.g. ammonia or other volatile base such as an alkylamine or arylamine, also hexamethylenetetramine or its salts, calcium carbonate, zinc oxide or caustic soda solution.  The solid formaldehyde may be separated by filtration, suction or centrifuging, and should advantageously be dried immediately in the shortest possible time, e.g. at 30--40 DEG  C. under 100 mms. pressure.  In examples: (1) the hot reaction gases obtained in the catalytic oxidation of methyl alcohol are condensed in a receiver kept at 62--67 DEG  C. by a cooling coil, the uncondensed vapours being worked up in a connected absorption plant; the condensate, consisting of formaldehyde, methyl alcohol, water and a trace of formic acid, is discharged at about 65 DEG  C. into a crystallizing vessel insulated against radiation, where, after addition of a small quantity of ammonia, it cools to 15 DEG  C. in 20 hours, the precipitate which forms being separated by filtration and dried in vacuo at 30 DEG  C.; (2) aqueous formaldehyde is distilled with ethyl acetate and the residual hot solution is cooled at about 3 DEG  C. per hour, the crystalline paste formed being filtered in an atmosphere of the ester, washed, if required, with pure ester, and dried as in (1).  The Specification as open to inspection under Sect. 91 comprises in general the manufacture of solid formaldehyde by subjecting formaldehyde solutions to a regulated slow cooling, which may be p  more rapid than that specified above, e.g. from 65 DEG  to 15 DEG  C. in 12 hours.  When working up solutions containing solvents other than water, e.g. ethyl acetate, to a preponderating extent, rates of cooling higher than those specified above may be employed. This subject-matter does not appear in the Specification as accepted.